This invention relates to a conductive, acoustically permeable housing, which provides electrical shielding and mechanical protection of a microphone transducer.
Microphones often utilize transducers which operate at very high impedance and low signal amplitude, requiring shielding to prevent external electric fields from being coupled into the microphone circuit. The transducers are often physically fragile, because high compliance in the moving structure is required to produce good performance. These two factors together require a robust housing to enclose the transducer. Such a housing, when it is robust enough to provide the needed protection, often has undesirable acoustical influence on the performance the transducer due to reflections from solid surfaces that make up the housing.
It is well known to enclose a microphone transducer in a basket or grille of one or more layers of perforated metal or wire cloth. Because of the flexible nature of this material, it is often attached to a support structure of solid material. Sound arriving at the microphone may pass through the open grille or may strike and be scattered by its support structure. Once sound is inside the housing, it may be reflected multiple times between parallel or coaxial surfaces. This scattering and reflection may result in anomalies in frequency and phase response of the microphone due to interference between the original sound waves and the scattered or reflected waves.
Previous attempts to resolve this problem have concentrated on reducing the use of parallel or coaxial surfaces that might give rise to internal reflections within the housing. Common shapes have included slanted, tapered and irregularly shaped surfaces. Additional structure, which creates its own reflections, is needed to support the housing, since most of the acoustically permeable grille or basket is of flexible mesh or woven material.
It is also known, for example in the AKG D-202 microphone made in the 1960s, and as described recently in US Patent Application 2007/0003095 to employ sintered plastic or metal to provide mechanical protection and wind screening, but such sintered material, being made of substantially spherical grains bonded together, typically has an open area of less than 50% and as such produces significant alteration in the sound due to its flow resistance.
An additional problem is that microphones are sensitive to wind and breath noises, which cause undesirable degradation in the sound pickup. It is also known to provide wind and pop shielding for microphone transducers by enclosing them within a body of reticulated, open cell plastic foam, as taught in U.S. Pat. No. 3,236,328 to Lou Burroughs. Such wind and pop screening material provides no electrical shielding, and is subject to degradation as the plastic ages.